Most power transmission parts (for example, a gear, a bearing, a CVT sheave, a shaft, and the like) used for automobiles, construction machines, farm machines, electricity generating-wind turbines, other industrial machines and the like are used after being subjected to a surface hardening treatment to improve, for example, fatigue characteristics and abrasion resistance of the parts. Among a plurality of known surface hardening treatments, a carburizing treatment is superior to other surface hardening treatments in terms of the surface hardness, depth of hardened layer, productivity, and the like, so the carburizing treatment is applied to numerous types of parts.
For example, in a general manufacturing method of the gear and bearing parts, by using medium carbon alloy steel such as SCM 420, SCR 420, SNCM 220, and the like specified by JISG 4053, a mechanical processing is performed to obtain a predetermined shape through a hot forging, a cold forging, cutting, or through a combination thereof, and then the carburizing treatment or a carbonitriding treatment is performed.
The fatigue fracture of a carburized gear is classified roughly into bending fatigue (dedendum fatigue) and tooth surface fatigue (pitting or the like). In order for the gear parts to obtain durability, both the above types of fatigue strength need to be improved. Among these, it is known that the bending fatigue strength can be improved by improving the surface microstructure (reducing a grain boundary-oxidized layer, and an incompletely quenched layer), and can be drastically improved by performing shot peening.
On the other hand, the tooth surface fatigue strength can hardly be improved even if the shot peening is performed, so the improvement of the tooth surface fatigue strength is anticipated. Moreover, since a high contact pressure is applied not only to the gear but to the CVT sheave and bearings, there is an eager request for the improvement of surface fatigue such as tooth surface fatigue and rolling fatigue.
Recently, it has been reported that hardness at 300° C. (or hardness after tempering at 300° C., which is referred to as tempered hardness at 300° C. hereinafter) should be increased in order to improve effectively the tooth surface fatigue strength, since the temperature of the tooth surface rises up to about 300° C. while using the gear.
It has been suggested that the tempered hardness at 300° C. of steel is improved by increasing the amount of Si or Cr added. However, even if the chemical composition of steel is optimized, the improvement of the tempered hardness at 300° C. has a limit and its drastic improvement is difficult. Therefore, another technique for the improvement is anticipated.
Recently, as a technique of drastically improving the tempered hardness at 300° C., a “high carbon carburizing technique (which is also referred to as high concentration carburizing)” which intentionally precipitates cementite on the surface of a carburized part has been suggested. In the general carburizing treatment, the atmospheric carbon potential (hereinafter, referred to as C. P.) is set to about 0.70% to 0.90%, the carbon content in the surface layer portion of the part is controlled to be about 0.80%, and then quenching is performed. As a result, the microstructure in the surface layer of the part is controlled into a martensite phase containing about 0.80% of carbon by the treatment.
On the contrary, in the high carbon carburizing technique, by setting the C.P. to a higher level (a level equal to or higher than eutectoid carbon content) compared to the general treatment, the carburizing treatment which results that the carbon content in the surface exceeds an Acm composition is performed. As a result, since cementite precipitates and disperses in the surface layer portion, a microstructure which has the cementite dispersed in martensite is finally obtained. This technique is also called CD carburizing: carbide dispersion carburizing.
“Carbon potential” is a term indicating the carburizing ability of the atmosphere in which steel is heated. The basic definition of carbon potential is “carbon content of the steel surface which is in equilibrium with a gas atmosphere after the steel is carburized by heating at a certain temperature under the gas atmosphere.” Here, in the case that carburizing is conducted under a condition such that the value of the carbon potential exceeds the Acm composition of carburized steel, the carbon potential means the hypothetical carbon content of the steel surface where cementite precipitation does not occur. In this case, the carbon potential is not necessarily identical to the actual amount of solute carbon of the steel surface.
Exposed to a high temperature in use as the part, martensite is tempered and softened. On the other hand, carbide such as cementite or the like is much harder than martensite and not easily softened even when the temperature rises. Consequently, if a large amount of the carbide can be dispersed in steel, it is possible to drastically improve the tempered hardness at 300° C. described above. This technique is hopeful method of improving the surface fatigue strength such as tooth surface fatigue strength, rolling fatigue strength, or the like.
However, when the high carbon carburizing is performed on medium carbon alloy steel such as SCM 420, SCR 420, SNCM 220, and the like specified by JISG 4053, coarse cementite precipitates inevitably along the austenite grain boundary (so-called pro-eutectoid cementite) and becomes a site where fatigue cracks initiate and a path of propagating the cracks. Therefore, the bending fatigue strength decreases and the tooth surface fatigue strength is unstable. As a result, the problem such that the expected fatigue strength is not obtained occurs.
For this reason, in the general high carbon carburizing, the high carbon carburizing is performed at a relatively high temperature (primary carburizing) at first, and then the carburized steel is temporarily cooled to around room temperature at a sufficiently rapid cooling rate so as not to precipitate the pro-eutectoid cementite. Thereafter, the carbide precipitating treatment which is reheat of the carburized steel to a temperature range coexisting austenite and cementite is carried out, and then quenching is performed. This treatment is generally called “secondary quenching”.
For example, Patent Document 1 suggests a method of performing the induction hardening as the secondary quenching. Patent Document 2 suggests a method to disperse carbide finely by specifying a heating pattern of the secondary quenching. However, the secondary quenching not only raises the cost of the heat-treatment but worsens a performance of the part. That is, the shape change of the part caused by the heat-treatment accumulates and increases inevitably by repetition of the heat-treatment. Therefore, the dimensional accuracy of the part deteriorates.
For example, if the dimensional accuracy of parts of the gear and bearing deteriorates, the part causes an increase in noise and vibration, because the parts are put into a unit such as a transmission unit and the unit is driven. In order to restore the dimensional accuracy of the parts, after the secondary quenching, cutting work is performed again in some cases (finishing processing).
However, since the cutting work is performed on the part including a surface layer portion which is extremely hardened by the high carbon carburizing, the cutting work is very difficult and inefficient, and the cost rises. Moreover, since a portion of the surface layer is removed through the finishing processing, it is necessary to form an extra hardened layer as deep as the cut portion. Accordingly, the carburizing treatment should be performed for many hours, and therefore, the productivity of the carburizing treatment decreases.
That is, in the current technique, although the secondary quenching should be performed to obtain the desired performance, the secondary quenching causes various problems, and as a result, a cost rises significantly.
In order to solve the above problems, materials for high carbon carburizing and a method of high carbon carburizing treatment have been suggested.
Patent Document 3 and Patent Document 4 suggest a method of manufacturing the part applied to high contact pressure in which a surface carbon content in the carburizing treatment is specified, the carburizing treatment is performed in a temperature range calculated by a formula specified by chemical composition, and a diffusion treatment is performed at high temperature which relates to the carburizing temperature after the carburizing treatment.
However, since the diffusion treatment is performed at a temperature higher than the carburizing temperature, the carbide is coarsened. Therefore, this method is unsuitable as a treatment replacing the secondary quenching. In addition, in the treatment pattern disclosed as an example of the invention of the above patent literature, steel is cooled rapidly to a point A1 or lower during the carburizing treatment. This is substantially the same treatment as the secondary quenching, and this method cannot prevent the deterioration of the dimensional accuracy of the parts.